Ethylene-.alpha.-olefin rubbers such as an ethylene-propylene copolymer rubber (EPM) which is a random copolymer of ethylene and propylene and an ethylene-propylene-diene copolymer rubber (EPDM) which further contains ethylidenenorbornene as diene have molecular structures containing no unsaturated bond in the main chains and are superior to general-purpose conjugated diene rubbers in heat resistance and weathering resistance. Therefore, they have been widely used as materials of automobile parts, electrical wire materials, building and civil engineering materials and industrial materials.
The ethylene-.alpha.-olefin rubbers are generally used after vulcanized, and the properties of the vulcanized rubbers vary depending on ethylene content, molecular weight, iodine value, etc., so that the rubbers different in these property values are appropriately used according to use application.
For example, use of EPM or EPDM having a high ethylene content makes it possible to obtain vulcanized products of high heat resistance, or use of EPM or EPDM having a low ethylene content makes it possible to obtain vulcanized products of good low-temperature flexibility.
By the way, among various uses of rubbers, rubber vibration insulators such as brake parts and engine mounts are desired to have particularly good heat resistance and low-temperature flexibility.
However, even if the values of ethylene content, molecular weight, iodine value, etc. are varied, it is difficult to obtain vulcanized products satisfactory in heat resistance and low-temperature flexibility required for the rubber vibration insulators from the conventionally known EPM or EPDM. Therefore, EPM or EPDM has been rarely applied to such uses.
Accordingly, now desired is the advent of the ethylene-.alpha.-olefin-nonconjugated polyene copolymer and vulcanized products thereof which are improved in the aforementioned problems, and excellent in heat resistance and low temperature flexibility.
Though the advent of the copolymer, especially ethylene-.alpha.-olefin (C.sub.4 or more) -nonconjugated polyene copolymer is desired, such copolymer has not been existed until now.
The ethylene-.alpha.-olefin-nonconjugated polyene random copolymer (EPDM) are conventionally prepared by the use of vanadium type catalyst. However, it is difficult to industrially prepare an ethylene-C.sub.4 or more .alpha.-olefin-nonconjugated polyene random copolymer by the use of the conventionally known vanadium catalyst, because if ethylene, an .alpha.-olefin of 4 or more carbon atoms and a nonconjugated polyene, e.g., ethylidenenorbornene (sometimes referred to as "ENB" hereinafter) or 7-methyl-1,6-octadiene (sometimes referred to as "MOD" hereinafter), are copolymerized in the presence of the vanadium catalyst, the molecular weight is not raised and the polymerization activity is markedly lowered as compared with the case of preparing the EPDM.
In the case where ethylene, an .alpha.-olefin such as propylene and the ENB or MOD are copolymerized in the presence of a solid titanium catalyst known as a catalyst for preparing polyethylene or polypropylene, the resulting copolymer has a wide composition distribution and shows poor vulcanization properties. Moreover, in the solution polymerization process, a component of high ethylene content is precipitated, and therefore it becomes difficult to perform polymerization in the uniform solution state.
Japanese Patent Laid-Open Publication No. 51512/1990 discloses an unsaturated ethylene-.alpha.-olefin random copolymer which can be subjected to high-speed vulcanization differently from the conventional unsaturated ethylene-.alpha.-olefin random copolymers, and Japanese Patent Laid-Open Publication No. 64111/1990 discloses a process for preparing EPDM of low crystallinity and high molecular weight using a catalyst comprising a metallocene compound of titanium, zirconium or hafnium and aluminoxane by a slurry polymerization.
In either publication, however, there is no description on the preparation of an ethylene-.alpha.-olefin-nonconjugated polyene random copolymer having a narrow composition distribution and excellent in mechanical strength, low-temperature flexibility and heat aging resistance by copolymerizing ethylene, an .alpha.-olefin of 4 or more carbon atoms and a nonconjugated polyene with high activity.
Further, in the case of copolymerizing ethylene, .alpha.-olefin of 3 or more carbon atoms and nonconjugated polyene using the conventionally known catalyst, there arise such a problem that ethylene-.alpha.-olefin-nonconjugated polyene random copolymer containing the .alpha.-olefin of 3 or more carbon atoms in high content is hardly obtained since the reaction ratio (conversion ratio) of .alpha.-olefin of 3 or more carbon atoms is lower than ethylene.
Accordingly, now desired is a process for preparing an ethylene-C.sub.3 or more .alpha.-olefin-nonconjugated polyene random copolymer in which ethylene, an .alpha.-olefin of 3 or more carbon atoms and a nonconjugated polyene can be copolymerized with high activity, and high conversion ratio of .alpha.-olefin, and an ethylene-.alpha.-olefin-nonconjugated polyene random copolymer having a narrow composition distribution, high molecular weight and excellent in low-temperature flexibility and heat aging resistance can be prepared.